Intermittently driven tubular-bag machines commonly operate in such a manner that a thin foil of a hot-sealable material is pulled off from a storage roller and the edges thereof guided together at a forming pipe to form a tube, after which the edges are welded together to form a longitudinal seam and the created tube is provided with sealing seams extending transversely with respect to the direction of transport. A tubular bag, which is closed at its bottom end, is supplied with packing material through the forming pipe and is then sealed at the top end (see U.S. Pat. No. 4,750,313.
In the common tubular-bag machines of this type, the drives of which operate with constant rotation frequencies, high dynamic stresses are created in view of the industrially requested performances, mainly stresses on the foil removing and cross-sealing devices. The proportionate times for the sealing task are during very short work cycles so short that the quality of the cross seams is lacking, for example, due to an insufficient heating up of the foil layers and insufficient cooling or solidification of the seam. Also the relatively early stress applied to the seam caused by the bag filling material can have negative effects. A tensile stress occurs on the filler pipe itself due to a frictional engagement of the lateral foil removing bands or belts, which tensile stress can result in an excessive stretching of the foil which results in an inferior package quality. In cases where only one single drive motor is used, the thus needed coupling-braking combination is subjected to a considerable amount of wear.
A main source of stress for the packaging machine is the common cam control structure for the sealing tools. Such high acceleration forces occur that the rollers can lift off from the control curves or cam surfaces in the area of extreme curvatures of the cam surfaces. It is also disadvantageous during adjusting and/or setting up of the machine for the angle adjustment of the control curves or cam surfaces to the filling and folding times of the packing material and to the foil removing times to have to be done very precisely, because such automatically requires much time.
An important goal of the invention is to improve the drive of the tubular-bag machines in a simple manner such that the sequences of movement are carried out as smoothly as possible and with little or no acceleration. Furthermore, the drive is to be flexibly controllable in order to substantially do justice to the practical operation demands and to achieve a high packing performance. It is further more desired to reduce the structural expense of the driving system.
Two separate, however, cooperating drives are provided according to the invention for effecting the movement of the foil on the one hand and of the sealing jaws on the other hand, which drives are in particular computer controlled. Due to these measures a great quietness and high degree of flexibility of the drive is achieved. Optimum sequences of movement with very little expense can be achieved for many different uses. Performance ranges are thereby achieved also for intermittent tubular-bag machines, in which up to now a continuous operation was necessary. The stress limits are now not only determined by the machine elements, but by the characteristics of the diverse packing materials.
This is substantially helped if, according to one measure of the invention, the angular speed of the control curves or cam surfaces for the sealing jaws is variable. Thus it is possible for at least two angular-speed stages to occur within each work cycle, which pass preferably smoothly over into one another and can have an extreme relationship of up to 1:5. From this do not only result great advantages for the method, but also for the equipment and inventory of parts, since one is now able to work with uniform control cams of few basic forms and sizes.
Two similar control cams sit in a preferred embodiment offset at 180.degree. to one another on a drive shaft, the speed of which is variable during each rotation. A special design of the control cam is particularly advantageous both for minimizing the dynamic stresses and also for creating optimum sealing seams, because the angular speed is always the lowest in the sealing phase, even though the associated curved sector can be limited to a relatively small center angle of, for example, approximately 60.degree.. This allows for very favorable kinematic transitions to other radii or stroke positions, which extend over large angle ranges.
The method and device of the invention are constructed such that optimum long durations and cooling times are available whereat a cross-sealing device is provided. Furthermore, the foil transport is limited to a portion of the work cycle, so that not only a protective removal of the foil material is assured, but simultaneously also the packaging and foil removal is substantially increased. With this a much more flexible operation of intermittent tubular-bag machines is achieved in comparison to the state of the art, which machines are thus considerably more versatile and more efficient than common machines.